Jean-Marie Dewarumez

First Record of the American Jack Knife Clam Ensis Directus on the French Coast of the North Sea

Ensis directus (Bivalvia: Solenidae) was noted for the first time along the French coast of the North Sea in June 1991. High numbers of post larvae after a period of northerly winds indicate that settled larvae originated from adult populations in Belgian or Dutch coastal waters. Future dispersal of Ensis directus could be used as a biological tracer of coastal water movements between the Southern Bight of the North Sea and the English Channel. The American jack knife clam, Ensis directus (Conrad, 1843), synonym Ensis americanus (Binney, 1870) sensu van Urk (1964,1972), was discovered in Europe in the German Bight for the first time in June 1979 (Von Cosel et al. , 1982). This species is assumed to have been transported in its larval stage by a ship containing ballast water. Considering the dimensions of the specimens, this probably happened in the first half of 1978 (Von Cosel et al. , 1982; Muhlenhardt-Siegel et al. , 1983). Since then this species has spread rapidly in the North Sea in subtidal and intertidal areas. Dense populations were found along the German coast within a few years (Von Cosel et al. , 1982; Muhlenhardt-Siegel et al. , 1983; Swennen et al. , 1985). In 1986 Ensis directus was reported from the north and east Danish coasts as far as the Belgian coast (Kerkhof & Dumoulin, 1987) (Figure 1). Since then no new records have been reported.

North Sea ecosystem change from swimming crabs to seagulls.

A recent increase in sea temperature has established a new ecosystem dynamic regime in the North Sea. Climate-induced changes in decapods have played an important role. Here, we reveal a coincident increase in the abundance of swimming crabs and lesser black-backed gull colonies in the North Sea, both in time and in space. Swimming crabs are an important food source for lesser black-backed gulls during the breeding season. Inhabiting the land, but feeding mainly at sea, lesser black-backed gulls provide a link between marine and terrestrial ecosystems, since the bottom-up influence of allochthonous nutrient input from seabirds to coastal soils can structure the terrestrial food web. We, therefore, suggest that climate-driven changes in trophic interactions in the marine food web may also have ensuing ramifications for the coastal ecology of the North Sea.

Is the ‘muddy heterogeneous sediment assemblage’ an ecotone between the pebbles community and the Abra alba community in the southern bight of the North Sea?

Abstract Three assemblages of the Southern Bight of the North Sea seem to constitute a special system of two adjoining communities separated by an ecotone. The specific trait of the middle assemblage, established on a muddy heterogenous sediment, resembles the pebbles community in its western part and the Abra alba community in its eastern part. The pebbles community is dominated by epifauna (sessile and mobile species) and the Abra alba community is dominated by infauna. The bionomic and trophic structures of the ‘muddy heterogenous sediment assemblage’ are a combination. This assemblage is composed of suspension feeders and deposits feeders, whereas the pebbles community is dominated by suspension feeders and the Abra alba community by deposit feeders. A comparison of the development of the Abra alba and the muddy heterogenous assemblages during an eleven-year period shows that both assemblages have evolved in the same way. They show a bionomical stability during the same period, simultaneous bionomical changes, and recruitments of the same species at the same time. These parallel evolvements indicate that the muddy heterogenous sediment assemblage can be considered an ecotone between the Abra alba and the pebbles communities.

Warm-water decapods and the trophic amplification of climate in the North Sea

A long-term time series of plankton and benthic records in the North Sea indicates an increase in decapods and a decline in their prey species that include bivalves and flatfish recruits. Here, we show that in the southern North Sea the proportion of decapods to bivalves doubled following a temperature-driven, abrupt ecosystem shift during the 1980s. Analysis of decapod larvae in the plankton reveals a greater presence and spatial extent of warm-water species where the increase in decapods is greatest. These changes paralleled the arrival of new species such as the warm-water swimming crab Polybius henslowii now found in the southern North Sea. We suggest that climate-induced changes among North Sea decapods have played an important role in the trophic amplification of a climate signal and in the development of the new North Sea dynamic regime.

Climate-induced range shifts of the American jackknife clam Ensis directus in Europe

Mapping the future potential distribution of alien species has become an issue of great concern. Ecological niche models are increasingly used to forecast the spatial range of introduced species in the context of climate warming. Here, we studied the potential spread of the American jackknife clam Ensis directus into European waters. E. directus, a marine bivalve native to the American coasts, was observed in Europe for the first time in the German Bight at the end of the 1970s. Afterwards, the clam quickly colonized the surrounding waters of the North Sea. Although many studies focused on its biology, ecology and colonization, the extent to which E. directus may invade European and Nordic seas remained poorly known. In this study, we used two ecological niche models (ENMs), calibrated on the native area of the mollusk, to evaluate the potential distributional range of the bivalve over European seas. Under current environmental conditions, E. directus should continue to progress towards the southern coasts of France and may also invade new areas in the Adriatic Sea. Projections for the end of the century suggest that the probability of occurrence of E. directus increases from Denmark to France with both ENMs. The Tunisian coasts may also become a new suitable area for the mollusk but the results of the two ENMs differ for this region. Therefore, contrary to what is often observed, a southward range expansion of E. directus is probable, especially as climate will get warmer.

Long-term changes (1979-1994) in two coastal benthic communities (English Channel): analysis of structural developments

Abstract The long-term variations of two coastal benthic communities (Eastern English Channel, France) were studied between 1979 and 1994 by means of factor correspondence analysis (FCA), trophic structure and rank-frequency diagrams (RFD). FCA identified periods of relative homogeneity and periods of strong variation in the structure of the two communities. The first community, established on heterogeneous sand, showed a multi-annual cycle linked to grain-size variations in the sediment. As passive supplies of organic matter were not important and suspension feeders could hardly live in these conditions, the community remained poor and stable and was dominated by deposit-feeders and carnivores. The other one, a rich and diverse mussel ( Mytilus edulis ) bed, showed great variations in the abundance of the main species which generated the enrichment of the community by its suspension-feeding behaviour (biodeposition). Beyond these variations, the community remained stable until 1990 as no continuous trend or permanent change could be seen. Since 1990, no recruitment of M. edulis occurred and the relative dominance of suspension feeders, as well as the number of species, strongly decreased. During the following years, several species of tunicates (sessile suspension feeders with gregarious recruitment) recruited and partly took the ecological niche of M. edulis but they did not induce an increase of biodeposition and the previous richness was not restored. After a strong and rapid shift in the structure, a new equilibrium has been established within the community.

Long-term changes (1979–1990) in three benthic communities (eastern English Channel): Use of factor analysis and rank-frequency diagrams for studying structural developments

Long-term variations (1979–1990) of three benthic communities (Eastern Channel, France) were analysed by means of factor correspondence analysis (FCA), a multivariate method of ordination in reduced space, and rank-frequency diagrams (RFD). FCA distinguished periods of relative homogeneity and periods of strong variations in the structure of the communities. Two communities, the pebbles community and a rich and diverse mussel (Mytilus edulis) bed, showed great variations in the abundance of the main species (Ophiothrix fragilis in the first one, andM. edulis in the second one) which generated the enrichment of the community by their suspension-feeding behaviour. Beyond the multi-annual cycle induced by these variations, both communities seemed to be quite stable as no continuing trends could be seen. The third community, established on heterogeneous sand, also showed a multi-annual cycle linked to granulometric variations of the sediment. As passive supplies of detritus were not important and suspension feeders could hardly live in these conditions, the community stayed poor and was dominated by carnivores and deposit-feeders.

Population dynamics in lesser black- backed gulls in the Netherlands support a North Sea regime shift

Shamoun-Baranes & Camphuysen [1] made two main points in their critical appraisal of our recent article [2]: (i) that the Larus fuscus population increased in the Netherlands well before a mid 1980s regime shift in the North Sea and (ii) that population increases based on a simple prey type are difficult to imagine. These two comments give us the opportunity to deepen and complete our conclusions. Before we consider them, however, it is important to note that Shamoun-Baranes & Camphuysen [1] present conclusions outside the time window considered in our study, focus only on the Netherlands, and do not give any analysis of population trends. With respect to the first point—that the L. fuscus population increased in the Netherlands well before a mid 1980s North Sea regime shift—it is important to highlight that the regime shift we referred to occurred in the mid-1990s. As our figure 2 [2] indicates clearly, the gull colonies in the Netherlands changed little during our study period from 1986 to 2000. Consequently, the first point made by Shamoun-Baranes & Camphuysen [1] has to be viewed as an idiosyncratic characteristic when considering spatial heterogeneity as a fundamental ecological property in ecosystem functioning [3], for which our analysis of gull populations in the broader North Sea region, allowed. With respect to the second point—that population increases based on a simple prey type are difficult to imagine—Shamoun-Baranes & Camphuysen [1] support their argument with the observation that crustaceans have much lower energy value than fishes, which represented around 80 per cent of the gulls diet by mass. While we agree (even though we did not find these results in their cited reference [4]), the same study [4] reports that the swimming crab Liocarcinus holsatus is the third most important prey for L. fuscus in the Texel colony (20% in the 2005–2010 period), confirming de facto part of our findings. As shown by several authors [5,6], seabirds may be influenced by changes in both food quantity and quality, and they may affect different components of reproduction and survival: for example, fledging success (survival from hatching to fledging) shows a positive relationship with food quality, but not with food quantity [5]. In this regard, Shamoun-Baranes & Camphuysen [1] omitted to mention a key study by Schwemmer & Garthe [7], which showed that swimming crabs—Liocarcinus sp.—were a major dietary item during both the egg-stage and the chick-rearing period, when the number of breeding pairs of L. fuscus were increasing exponentially along the German coast in the 1990s (figure 2 in [2]). During the breeding season, birds require specific nutrients in their diet [8] and Schwemmer & Garthe [7] suggested that swimming crabs—Liocarcinus sp.—may be a valuable source of calcium for both eggshells and the bone development of chicks. Interestingly, the observations by Schwemmer and Garthe may have also provided an answer to Camphuysen [9], who concluded that the foraging range of L. fuscus further out to sea could not be explained fully by either a change in the abundance of fishing vessels or the avoidance of herring gulls (Larus argentatus), and that it must, therefore, involve another unknown offshore food resource. Shamoun-Baranes & Camphuysen [1] only referred to their own local study [9] on the importance of fishing discards in the gulls diets, whereas swimming crabs found in the L. fuscuss diet are obtained by natural feeding [4,7]. One hypothesis proposed by Schwemmer & Garthe [7] to support their findings was an increase in the abundance of Liocarcinus sp., which we confirmed in our study [2]. To conclude, the observations of Shamoun-Baranes & Camphuysen [1] on a single population of L. fuscus from the Netherlands most likely indicate that they were at the wrong place (idiosyncratic results owing to spatial scaling and heterogeneity), at the wrong time period (ante 1989), while also failing to consider food quality as an important parameter in food selection. Although Lindley et al. [10] showed that the increase in decapods (dominated by swimming crabs of the sub-family Polybiinae) was a key component of the trophic amplification of hydroclimatic change and the development of a new North Sea ecosystem dynamic regime [11,12], we do agree with Shamoun-Baranes & Camphuysen [1] that a deeper exploration of the drivers influencing gull population dynamics at several spatial and temporal scales is needed, as it is for any complex adaptive system.